1. Field of Invention
This invention relates to a method for controlling the position of a movable object and a positioning system for a lithographic apparatus. More particularly, the invention relates to a method and a positioning system for controlling a short stroke module of a substrate stage or reticle stage of a lithographic apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, such as, for example, a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The known lithographic apparatus includes a number of movable objects, which have to be actuated with relatively large accelerations and high accuracy. An example of such a movable object is a reticle stage, which supports a reticle during various lithographic processes. Another example is a wafer stage, which supports a wafer during the projection process. Such a wafer stage may include a long stroke module and a short stroke module. The long stroke module supports the short stroke module, while the short stroke module is configured to carry the wafer. The long stroke module is used for coarse positioning of the wafer stage, while the short stroke module is used for fine positioning of the wafer with respect to the projection system. In the known lithographic apparatus, a Lorentz motor is used for the actuation of the short stroke module. Such a Lorentz motor is known to be a very accurate motor with which the short stroke module can be placed with high accuracy in the correct position. However, Lorentz motors are not highly efficient and a relatively large amount of heat is dissipated during the use of the motor. Such dissipation of heat is generally undesirable, in particular when large forces have to be produced by the motor during acceleration of the short stroke module. The generated heat may have a negative influence on the control accuracy of the Lorentz motor and on the performance of other systems in the lithographic apparatus.
It has been proposed to provide two types of motors for the actuator arrangement of the short stroke module, one type having a high efficiency but being relatively inaccurate, the other type being highly accurate, but having a low efficiency. For example, U.S. Pat. Appl. Publication No. 2006/0061218 A1 proposes an actuator arrangement including two types of motor for the position control of a short stroke module. In this actuator arrangement, the use of E-I actuators (a type of reluctance motor that has high efficiency but is relatively inaccurate) is proposed to generate acceleration and deceleration forces to bring the short stroke module in the vicinity of a desired position. When the short stroke module is brought in the vicinity of the desired position, the control is taken over by a voice coil motor (a type of Lorentz motor that is highly accurate but has low efficiency), which is used to generate acceleration and deceleration forces to precisely control the position of the short stroke module. Thus, for the short stroke module there is carried out a coarse positioning phase in which the short stroke module is actuated by a first type of motor and a fine control phase in which the short stroke module is actuated by a second type of motor. Between these phases the two types of motors are switched off and on. As another example, U.S. Pat. No. 7,352,149 B2 proposes another actuator arrangement including two types of motor for the position control of a short stroke module in which the control involves using a feed-forward signal for a first motor assembly and a feedback signal for a second motor assembly. In this example arrangement, the first motor assembly can include any type of reluctance motors.
The use of reluctance motors in a positioning device is advantageous because they add less mass and dissipate less heat than other types of motors. However, the use of reluctance motors can behave nonlinearly and can also cause a parasitic damping effect.